I live in Springfield and took the LSAT at WNEC, so maybe I can help you out. The Greyhound station is, I'll estimate, 15 minutes from WNEC IF THERE IS NO TRAFFIC. Weekday mornings there can be major traffic in the downtown area, which is between the bus station and WNEC. I'd allow more time to be on the safe side. And better early than late.
I can't imagine the cab fare will be all that expensive, but then I haven't taken a cab in years so I really don't know. But the trip isn't more than a few miles.
The good news is, once you get to WNEC the law building is easy to find. The school is at the intersection of Wilbraham and Bradley roads, and the law building is off of Bradley. There is a parking lot right by the law building.